Upgrade firebase-functions to V4.3+

# Problem

We need to start cutting over to use V2 of the Google Cloud Functions API (because Google is going to start turning off deploys of the old API ).

As a first step, we need to upgrade our `firebase-functions` library to at least `4.3.0` (the first version to support the V2 API) - this will give us a foundation from which we can start making this critical transition.

This issue doesn't cover actually converting the functions to use V2 - it's solely to get the codebase in a state where we *can* start cutting over to V2.

# Success Criteria
* The MAPLE repo is upgraded to use `firebase-functions` V4.3 or higher (note that this is currently used in the `package.json` inside the `functions` folder of the main MAPLE repo)
* All existing unit/integration/E2E tests pass

# Additional Info

We attempted this upgrade as part of the social layer work and ended up breaking the integration tests, so we created this separate ticket to cover this work. The issue we ran into involved `firebase-functions` failing to find `firebase-admin/auth`, so we may need to upgrade `firebase-admin` and/or other firebase-related library versions to make this fly.